Liquid crystal display devices and various other types of display devices are currently used in a variety of applications. In a general display device, a single pixel consists of three subpixels respectively representing red, green and blue, which are the three primary colors of light, thereby conducting a display operation in colors.
A traditional display device, however, can reproduce colors that fall within only a narrow range (which is usually called a “color reproduction range”), which is a problem. FIG. 9 shows the color reproduction range of a traditional display device that conducts a display operation using the three primary colors. Specifically, FIG. 9 shows an xy chromaticity diagram according to the XYZ color system, in which the triangle, formed by three points corresponding to the three primary colors of red, green and blue, represents the color reproduction range. Also plotted by crosses ×in FIG. 9 are the surface colors of various objects existing in Nature, which were disclosed by Pointer (see Non-Patent Document No. 1). As can be seen from FIG. 9, there are some object colors that do not fall within the color reproduction range, and therefore, a display device that conducts a display operation using the three primary colors cannot reproduce some object colors.
Thus, in order to broaden the color reproduction range of display devices, a technique that increases the number of primary colors used for display purposes to four or more has been proposed recently.
For example, Patent Document No. 1 discloses a liquid crystal display device 800 in which one pixel P consists of red, green, blue, cyan, magenta and yellow subpixels R, G, B, C, M and Ye representing the colors red, green, blue, cyan, magenta and yellow, respectively, as shown in FIG. 10. The color reproduction range of such a liquid crystal display device 800 is shown in FIG. 11. As shown in FIG. 11, the color reproduction range, represented by a hexagon of which the six vertices correspond to those six primary colors, covers almost all object colors. By increasing the number of primary colors for use in display in this manner, the color reproduction range can be broadened.
Patent Document No. 1 also discloses a liquid crystal display device in which one pixel P consists of red, green, blue and yellow subpixels R, G, B, Ye and a liquid crystal display device in which one pixel P consists of red, green, blue, cyan and yellow subpixels R, G, B, C, Ye. In any case, by using four or more primary colors, the color reproduction range can be broadened compared to traditional liquid crystal display devices that use only the three primary colors for display purposes. In this description, a display device that carries out a display operation using four or more primary colors will be referred to herein as a “multi-primary-color display device” and a liquid crystal display device that carries out a display operation using four or more primary colors will be referred to herein as a “multi-primary-color liquid crystal display device” (or simply a “multi-primary-color LCD”). Meanwhile, an ordinary display device that carries out a display operation using the three primary colors will be referred to herein as a “three-primary-color display device” and a liquid crystal display device that carries out a display operation using the three primary colors will be referred to herein as a “three-primary-color liquid crystal display device” (or simply a “three-primary-color LCD”).
A video signal input to a three-primary-color display device generally has an RGB format or a YCrCb format. A video signal in any of those formats has three parameters (i.e., is a so-called “three-dimensional signal”), and therefore, the luminances of the three primary colors (that are red, green and blue) for use to conduct a display operation are determined unequivocally.
For a multi-primary-color display device to perform a display operation, a video signal with a format for a three-primary-color display device needs to be converted into a video signal with more (i.e., four or more) parameters. Such a video signal representing four or more primary colors will be referred to herein as a “multi-primary-color video signal”.
When the colors represented by a video signal in a format for a three-primary-color display device (which will be referred to herein as a “three-primary-color video signal”) are expressed in four or more primary colors, the luminances of the respective primary colors are not determined unambiguously but their luminances have a lot of different combinations. That is why a three-primary-color video signal is not converted into a multi-primary-color video signal by only one method but by any of various other methods very arbitrarily (with a lot of freedom). Methods for converting a three-primary-color video signal into a multi-primary-color video signal are proposed in Patent Documents Nos. 2 and 3, for example. Naturally, however, they are not the only ones but there is a wide variety of methods available.